A new animal DNA species has been isolated from brain and other tissues. It is characterized by the presence of 5-hydroxymethylcytosine, a pyrimidine base previously considered to be present only in the DNA of certain bacteriophages. Glucose is also found in this brain DNA as in the nucleic acid of these bacteriophages. Several lines of evidence indicate this DNA species in brain to be fundamental to the steady state maintenance of higher central nervous system function: its composition alters with development; it is metabolically labile; it appears to be a target site in the action of certain addictive drugs. It has been found that the concentration of 5-hydroxymethyl-cytosine (5-HMC) in brain DNA rises sharply from the perinatal period to maturation. It is 8.25 percent of the total cytosines in the brain of the 2 day old rat, and 18.5 percent in the adult. DNA preparations isolated from corresponding liver samples show a decline in 5-HMC level in this maturation interval. The biochemical and physiological characteristics of this DNA species sugested that it might be affected by or involved in some forms of retardation. Young aaimals on a low protein diet develop with both a reduced brain weight and behavioral deficits. Using such rats as models, it is found that the low protein diet produces a massive inhibition of 5-HMC synthesis in the brain, or of the DNA species which it characterizes. The brain DNA content of 5-HMC in rats 2 months old is 8.6 percent of the total cytosines compared with 18.5 percent in age controls, 16.7 percent in weight control and 8.25 percent in normal 2 day old rats. Initial experiments indicate that 5-HMC is formed by modification of cellular DNA. Optimal conditions for synthesis will be estabished and the reactions will be studied in retarded animals and age and weight controls to ascertain the biochemical site affected by malnutrition. Turnover of 5-HMC in DN will be studied in these 3 groups. Fractionation will be carried out to determine whether 5-HMC is common to all forms of brain DNA or whether it is restricted to a particular species. The possible localization of 5-HMC in major areas and subcellular fractions of the brain will be investigated.